1. Technical Field
The present invention relates to a transistor, a method for manufacturing thereof, and a substrate for an electrooptical device.
2. Related Art
As an example of typical transistors, a thin-film transistor (TFT) is provided on an insulating substrate and includes a semiconductor film having source, channel and drain regions, a gate electrode, and drain and source electrodes coupled to the source and drain regions, respectively.
The TFT is used as a switching element for each pixel in a transmissive liquid crystal panel as a substrate for an electrooptical device.
To drive the transmissive liquid crystal panel, a high voltage of about 15 V needs to be applied to the TFT. However, the higher the voltage applied to the gate electrode, the more off-leakage current occurs depending on the voltage. It has been known that a lightly doped drain (LDD) region in which an impurity is dispersed in a low concentration between the channel region and the source/drain regions works effectively to reduce this off-leakage current, as well as LSI technology. JP-A-2003-257990 is an example of related art.
As for the transmissive liquid crystal panel, it is required to narrow the pitch between pixels, increase the aperture ratio, and provide a high-definition display. To produce a smaller TFT serving as a switching element included in each pixel, there has developed a way to reduce the length of individual gates to narrow the pitch between pixels.
However, to apply a high voltage as mentioned above to the gate electrode with the reduced gate length, it is necessary to enlarge the LDD region to increase an area available as an electric field. The need for the enlarged LDD region results in an insufficient reduction in the size of the TFT as a whole despite the reduced gate length.
It is therefore difficult to reduce the size of the TFT while maintaining the resistance to driving voltage.